Boiling water nuclear reactors operate for many years. Commencing with their initial construction and through their service lives, these reactors may accumulate debris in their closed circulation moderator systems. This debris can become an operating hazard if the debris is allowed to enter into the fuel bundle-containing core region. In order to understand this problem, a summary of reactor construction as it relates to the accumulation of debris in the core is described, as well as fuel bundle construction and the effects of debris entering into the fuel rod region of the fuel bundles.
Boiling Water Reactors are provided with a large, central core. Liquid water coolant/moderator flow enters the core from the bottom and exits the core as a water steam mixture from the top. The core includes many side-by-side fuel bundles. Water is introduced into each fuel bundle through a fuel bundle support casting from a high pressure plenum which is situated below the core. Water passes in a distributed flow through the individual fuel bundles, is heated to generate steam, and exits the upper portion of the core as a two phase water steam mixture from which the steam is extracted for the generation of energy.
The core support castings and fuel bundles are a source of pressure loss in the circulation of water through the core. This pressure loss assures the substantially even distribution of flow across the individual fuel bundles of the reactor core. There may be approximately 750 individual fuel bundles in a reactor core, so uniformity of flow distribution is important. To interfere with the pressure drop within the fuel bundles could affect the overall distribution of coolant/moderator within the fuel bundles of the reactor core.
The fuel bundles for a boiling water nuclear reactor include a fuel rod supporting lower tie plate assembly, in which the lower tie plate is a cast structure. The lower tie plate assembly includes at its lowest point a downward protruding bail covering an inlet nozzle. This inlet nozzle includes entry to an enlarged flow volume within the lower tie plate. At the upper end of the flow volume, there is located a rod supporting grid. Between the supporting grid and the nozzle there is defined a flow volume.
The rod supporting grid has two purposes. First, the rod supporting grid provides the mechanical support connection for the weight of the individual fuel rods to be transmitted through the entire lower tie plate to the fuel support casting. Secondly, the rod supporting grid provides a flow path for liquid water moderator into the fuel bundle for passage in between the side-by-side supported fuel rods.
Above the lower tie plate, each fuel bundle includes a matrix of upstanding fuel rods, which are sealed tubes each containing fissionable material which when undergoing nuclear reaction produce the power generating steam. At the upper end of the matrix of upstanding fuel rods is located an upper tie plate. This upper tie plate holds at least some of the fuel rods in vertical side-by-side alignment. Some of the fuel rods are attached to both the upper and lower tie plates. Between the upper and lower tie plates, there are usually included water rods to enhance water moderator to fuel ratio, particularly in the upper, highest void fraction region of the fuel bundle.
Fuel bundles also include about seven to nine fuel rod spacers at varying elevations along the length of the fuel bundle. These spacers are required because the fuel rods are long and slender, and would come into contact under the dynamics of fluid flow and nuclear power generation within the fuel bundles. The spacers provide appropriate restraints for each fuel rod at their respective elevations and thus prevent abrading contact between the fuel rods and maintain the fuel rods at uniform spacing relative to one another along the length of the fuel bundle for optimum performance.
Each fuel bundle is surrounded by a channel. This channel causes water flowing between the tie plates to be restricted to only one bundle in an isolated flow path between the tie plates. The channel also serves to separate the steam generating flow path through the fuel bundles from the surrounding core bypass region, this region being utilized for the penetration of the control rods. The water in the bypass region also provides neutron moderation.
In the operation of a boiling water nuclear reactor, maintenance of the originally designed flow distribution is important. From the lower (high pressure) plenum inlet to the core to the outlet from the core of the steam and water mixture through the upper tie plates of the fuel bundles, about 20 pounds per square inch (psi) of pressure drop is encountered at typical operating conditions. About 7 to 8 psi of this pressure drop occurs through the fuel support casting. This pressure drop is mainly to assure the uniform distribution of coolant/moderator flow through the many fuel bundles making up the core of the reactor and is related to the prevention of operating instabilities within the reactor at certain power rates of the reactor. At the lower tie plate of each fuel bundle, from the inlet nozzle into the flow volume and through the fuel rod supporting grid, about 1 to about 1½ psi pressure drop occurs which contributes to establishing flow distribution between the individual fuel rods of each fuel bundle. Finally, through the fuel bundle itself—from the lower supporting grid to the exit at the upper tie plate—about 11 psi of pressure drop usually occurs.
Typically debris within boiling water nuclear reactors can include extraneous materials left over from reactor construction. Corrosion during the reactor lifetime also liberates debris. Further debris can be introduced during the numerous outages and repairs of the nuclear reactor. Nuclear reactors include closed circulation systems that essentially accumulate debris with increasing age.
A particularly vexing and usual place for the accumulation of debris is in the fuel bundles between the fuel rods particularly in the vicinity of the fuel rod spacers. Debris particles tend to lodge between the spacer structure and the fuel rods and often dynamically vibrate with the coolant/moderator flow in abrading contact to the sealed cladding of the fuel rods. Such flow induced vibration within the reactor can cause fretting and eventually damage and/or rupture of the cladding of the fuel rods. The rupture of the cladding may lead to the undesirable release of fission gas accumulated and sealed within the fuel rod prior to the rupture. If sufficient fission gas has been released due to cladding ruptures, plant shutdown could be necessary.
Modern nuclear plants have both redundancy and many safety systems designed to counteract anticipated operating fuel failures, such as fuel rods becoming punctured by debris. As such, these failures may not affect safety; however, in almost all cases they result in the plant operating at less than optimum efficiency.